Coaxial tracking signal coupler for antenna feed horn

ABSTRACT

Tracking signals are extracted from the antenna feed horn by means of a two-stage coaxial coupler. The first stage consists of irises located on the horn end of a coaxial spear that extract signals in the first order circular transverse electric mode. The second stage consists of a probe located at the waveguide end of the coaxial spear that extracts TEM energy derived from signals in the first order transverse magnetic mode. Only one coupling stage is required for tracking circularly polarized signals, while both are required for tracking linearly polarized signals. In either case, fundamental mode transverse electric energy is passed without significant coupling or obstruction.

United States Patent [191 Rootsey COAXIAL TRACKING SIGNAL COUPLER FOR ANTENNA FEED HORN [75] Inventor: James V. Rootsey, Sunnyvale, Calif.

[73] Assignee: Philco-Ford International,

Philadelphia, Pa.

[22] Filed: Sept. 11, 1972 [21] Appl. No.: 287,708

[52] [1.8. CI 343/117, 343/786, 343/858 [51] Int. Cl G0ls 5/02 [58] Field of Search 343/l00 PE, I17, 786, 858

[56] References Cited UNITED STATES PATENTS 3,259,899 7/1966 Cook 343/858 3,566,309 21197,! Ajioka 343/786 3,568,204 3/l97l Blaisdell 343/786 3,665,481 5/]972 Lowet al 343/786 June 4, 1974 5 7] ABSTRACT Tracking signals are extracted from the antenna feed horn by means of a two-stage coaxial coupler. The first stage consists of irises located on the horn end of a coaxial spear that extract signals in the first order circular transverse electric mode. The second stage consists of a probe located at the waveguide end of the coaxial spear that extracts TEM energy derived from signals in the first order transverse magnetic mode. Only one coupling stage is-required for tracking circularly polarized signals, while both are required for tracking linearly polarized signals. In either case, fundamental mode transverse electric energy is passed without significant coupling or obstruction.

8 Claims, 6 Drawing Figures TMa/ 007/ 07 PATENTEDJUH 4:914 3815; 136

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aalsmas PATENTEDJUN 41924 SHEET 2 OF 2 i COAXIAL TRACKING SIGNAL COUPLER FOR ANTENNA FEED HORN BACKGROUND OF THE INVENTION Large ground-based communications antennas, particularly those for use with communications satellites, must be accurately pointed for reliable operation. Typically a large parabolic reflector, mounted on a frame that provides for rotation and declination of the reflector, is operated with a feed horn at its focus. The feed horn is shaped so as correctly to illuminate the large parabolic reflector in the desired manner. The transmission line coupled to the hornis usually a waveguide for low loss transmission. Alternatively, as in the Cassegrainian antenna, a sub-reflector is located at the antenna focus and the feed horn, located in a hole at the center of the parabolic reflector, illuminates the subreflector which in turn illuminates the parabolic reflector.

In still another version, a very large horizontal horn has a flat reflecting surface, tilted at a 45? angle with respect to the horn axis, located at the large end. The small end of the horn terminates in the waveguide transmission line. The horn can be rotated about its long axis and its-vertical beam center line to provide scanning or pointing.

Ordinarily such antennas are optimized for operation in a communications frequency band to provide reliable low-noise communication. Tracking is accomplished within this band and the satellite being tracked is ordinarily supplied with a beacon transmitter emitting at the tracking frequency for easy viewing. The beacon frequency is sufficiently off-set from any communications frequencies to avoid interference. In some systems, the communications antenna feed is surrounded by four tracking feed antenna horns, two each for azimuth and elevation data. Such a system is cumbersome and, since the communications feed is at the reflector focus, the tracking feeds must be compromised in terms of location and size. Ithas become standard practice to use a single feed horn and to extract the desired tracking signals from the waveguide transmission line. One successful prior art system is described in the article titled The Autotrack System published in the Bell System Technical Journal 42 1283-1307. (July, 1963).

The extraction of tracking signals is ordinarily by way of coupling irises inthe waveguide walls, or other externally mounted pick off elements, These elements are complex and must be located with great precision. The iris location, size, and coupling bandwidth must be compromised with respect to minimizing influence on the communications channel.

SUMMARY OF THE INVENTION It is an object of the invention to provide a coaxial waveguide coupling device to extract tracking signals.

It is a still further object to incorporate a probe into the coaxial coupling device to extract tracking signals with polarization in the plane of the pointing error.

It is a still further object additionally to extract reference signal information of any polarization by means of the waveguide containing the coaxial coupling device.

These and other objects are achieved by inserting a doubly tapered conducting coaxial element into the throat of the antenna feed horn where it joins the wave guide transmission line. The taper pointing toward the horn serves as a mode control section that will convert any TM, energy in the horn to TEM energy. The taperedsection pointing down the waveguide terminates in a coaxial probe. The coaxial probe extracts TEM energy from the coaxial section, thereby coupling out transverse magnetic signal energy from the horn. This energy contains antenna pointing error signal information whenever the field vector is in the plane ofthe pointing error. While such a device operates well on circularly polarized signals, it will not respond equally to all directions of linearly polarized signals.

The horn end of the coaxial element may also contain longitudinal slots or coupling irises that will be excited by rotary transverse electric signal energy (TE in the throat of the horn. All TE energy propagating into the horn will be reflected at the point of cut-ofl' and will set up a standing wave pattern. By placing the longitudinal slots at the field maximum, strong coupling to the circular electric mode occurs. For broadband operation a discrete step is added to the horn wall to define the cutoff point over the frequency band of operation. The

- TE mode is excited whenever the field vector of the received signal is perpendicular to the plane of the pointing error. This device also operates well on circularly polarized signals, but will not respond equally to all directions of linearly polarized signals.

In accordance with one feature of the invention, the limitations referred to in the two preceding paragraphs are avoided because the TE output is complementary in directional response to the TM output. When the modes are combined, the resulting output is nearly constant for all directions of linearly as well as circularly polarized signals.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a showing, in partial cross-section, of a coaxial tracking coupler located in the throat of an antenna feed horn;

FIG. 2 is a graph showing the receiving antenna mode response as a function of angle of arrival of incident signal;

FIG. 3 is a cross section of a tracking coupler which has provision for coupling to the TM input mode;

FIG. 4 is a cross section of a tracking coupler which has provision for coupling to the TE input mode;

FIG. 5 is a block diagram of a system using either the tracking coupler of FIG. 3, or the tracking coupler of FIG. 4; and

FIG. 6 is a block diagram of a system using both tracking couplers simultaneously as would be required for tracking a linearly polarized signal.

DESCRIPTION OF THE INVENTION FIG. 1 shows a cross section of a'feed horn used to excite a large parabolic antenna. The horn l is simply a flared section extending from round or square waveguide 2. The horn dimensions are selected to provide the desired parabolic reflector illumination as is well known in the prior art. The surface of the horn may be corrugated to aid in mode control if desired. The waveguide coupling to the horn is dimensioned to provide for operation at the TIE fundamental mode at the lowest frequency of operation. For the systems of interest this will usually be within 40 percent of the highest beacon or tracking frequency. Thus the waveguide will not support the higher order modes at the beacon frequency.

Coupler 3 is mounted inside the throat of the horn as shown. lts basic form is a tapered section of a conducting cylinder accurately centered with respect to the waveguide axis. This device is mounted by means of metal or low loss dielectric standoffs not shown in this view. These standoffs may be dimensioned and shaped to minimize r-f losses as is well known in the art. Coupling irises 41, see also FIG. 4, are the TE coupler and excite an inner coaxial line in the TEM mode. A small diameter coaxial cable '7 brings energy from the slot area to the exterior of the waveguide wall. Probe is the TM coupler and, as illustrated in FIG. 3, is connected to the center conductor of .a length of small diameter coaxial cable 6 that brings energy from the probe outside the waveguide wall where it is available for the tracking receiver. The horn end of coupler 3 includes a tapered section extending into the horn. This section serves to convert received signal energy in the TM mode to the TEM mode of coaxial line operation. While waveguide 2 will not support this mode, the coaxial line composed of inner conductor 3 and outer conductor 2 will propagate the TEM mode. The portion of coupler 3 acting on the wave approaching the horn to waveguide transition serves as a mode filter that tends to reject all modes except TE and TEM. The TE signals pass on down waveguide 2 but the TEM energy is matched by the taper to probe 5 which then conducts it out via coaxial cable 6.

If the source of the beacon signal is precisely on boresight (the antenna accurately pointed at the source) all energy will propagate down the horn l. and waveguide 2 in the fundamental TE mode, and there will be no TM. or TE excitation and no signal will be coupled out of coaxial cables 6 or 7. If, however, the source is located slightly off boresight, it will still propagate down waveguide 2 but it will also introduce a TlVl TE or combined component into the horn dependent upon its polarization. This component will appear at cables 6 or 7 (or both) and will have a phase relative to the TE signal dependent upon the direction of departure of the source from boresight.

FIG. 2 shows the relative performance of the TE and TM components as a function of boresight error. The TE component shows a broad maximum about the 0 or boresight angle. This constitutes a conventional plot of antenna beamwidth. The TM and TE. components go through a very deep null on boresight and rise to about 6 dB below the main beam value at relatively slight angular departure from boresight. The foregoing assumes that the beacon signal is completely circularly polarized. Any departure from this condition will result in variations in the TM and TE components as a function of the direction of departure from boresight.

FIG. 3 shows the details of coaxial probe 5. Coupler 3 is shown in cross section at the waveguide end. Probe 5 is connected by way of matching section 8 to the coaxial cable exiting the waveguide section as element 6. The probe assembly can be secured mechanically by filling the interior of the coaxial line with low loss dielectric material. The coaxial line 6 passes through the standoff 9 which may be a vane or rod of conducting or dielectric material.

FIG. 1 shows the horn end of coupler 3 containing four coupling irises 1 arranged in orthogonally disposed pairs. Since the drawing is a cross section of coupler 3, only two irises appear. Energy from these irises is coupled to the exterior of the waveguide by means of coaxial cable '7. The irises 4 will be excited only by TE energy. Since this mode will only propagate a limited distance down horn 11, step 10 is provided. The outer diameter of step 10 is made such that the horn will propagate TE energy, while the inner diameter of the step is beyond cutoff. Thus the step acts as a mode filter passing TE and TM energy but blocking TE energy. The irises 4 are located so that they will couple to the T13 field and, since this electric field is concentric with the horn, will be excited thereby. Iris energy is coupled by way of conventional coupling tabs 11 to coaxial line 12, which couples the TE energy out by way of cable 7. The inner conductor of coaxial line 12 is connected to the inner conductor of coaxial cable '7. Since the irises are excited in a circumferential manner in respect to the coupler, the fields must be reconfigured with respect to coaxial line 12 to excite it. This may be accomplished by slightly displacing opposing iris faces or by including tabs Ell. These tabs are fixed to one iris face and extend toward the center conductor of line 112. Each iris will have one tab and the tabs can be trimmed or adjusted to equalize the individual iris coupling.

The output from coaxial cable 7 will represent the TE input and will be available whenever the polarization is orthogonal to the plane of the pointing error.

FIG. 5 is a block diagram showing how either coupler may be connected into a tracking receiver for singlestage, circular polarization operation. The horn 1 and waveguide 2 feed a wideband low noise amplifier 13 and normally provides the communications signals to the receiver circuits. Tracking coupler 3 provides the TM signal by way of cable 6 or, alternatively, the TE signal by way of cable '7 to a phase lock tracking receiver M. The tracking receiver obtains its reference signal from the low noise amplifier by way of coupler 15. The tracking receiver 14 produces an output that contains sense information in terms of the phase difference of the two inputs to indicate the direction of departure from boresight, and the magnitude of the output indicates the amount of departure from boresight. These signals are then used in known manner to operate conventional antenna control servomechanisms 26 to minimize the boresight error. The mechanical linkage between antenna control servomechanisms 2t and horn 1 is represented by dashed line 22.

FIG. 6 is a block diagram showing how both stages of the coupler including the probe and irises of FIGS. 3 and 41 respectively might be connected to a tracking receiver for dual stage linear polarization operation. The iris coupling to the TE component is fed by way of cable 7 through a phase adjuster 16 and into one input port of a .hybrid coupler 18. The probe coupled TM component is fed by way of cable 6 to the other input port of coupler 118 through an amplitude adjuster 17. The phase adjuster control 16 compensates electrically for the physical separation between probe 5 and irises 4. The amplitude adjuster 17 equalizes the two signals. The signals are combined in the 90 hybrid coupler 18 for application to tracking receiver 14. From this point the operation is as was described for FIG. 5.

While the above describes a new tracking signal coupler, numerous equivalents will occur to a person skilled in the art. For example, the coaxial probe can take other forms such as a ball or disk, and conventional broadbanding techniques may be applied. The coupler and waveguide components, while being shown as circular in cross section, could be square or hexagonal. These elements could incorporate ridges for further control of transmission modes.

In place of the exciting tabs 11, the TE coupling slots could be excited by conducting loops, and the slots, rather than being rectangular, could be other shapes such as elliptical or dumbell. The means of propagating the TE signal to the slots could be accomplished by dielectric loading rather than a stepped horn. It is intended that the invention be limited only by the following claims:

1 claim:

1. An antenna system comprising: a beam steering system, a feed horn, a waveguide transmission line connectedto said feed horn, a coaxial coupler mounted at the juncture of said waveguide and said horn, said coupler containing elements responsive to non-axial horn signal components, said horn and waveguide being adapted to operate on TE, signal energy, said nonaxial components comprising a TE component when the departure from the axis is perpendicular to the polarization of said signal and a TM component when the departure from the axis is parallel to the polarization of said signal, said coupler including a plurality of longitudinal irises responsive to said TE, signal, said" communications and beacon signals from a satellite,

wherein said antenna includes means responsive to said beacon signals which, through the agency of antenna control servomechanisms, orient said antenna to track said satellite, said antenna including a horn section and a waveguide feed section, the improvement comprising: a coaxial coupler mounted at the juncture of said horn and said waveguide, said coaxial coupler including means extending into the throat of said horn section for converting TM horn signal components to TEM signal components, and means connected to said cou- Dler for extractin T .7.

and tracking ground station where the satellite to be tracked transmits communications signals at one frequency and beacon signals at a second frequency, said antenna comprising a horn section to receive both said signals and a waveguide section coupled to said horn section for transmitting received signals to signal utilization means, a coaxial coupler mounted within said horn and waveguide sections at the juncture thereof, one portion of said coaxial coupler extending coaxially into the throat of said horn and another portion of said coaxial coupler extending coaxially into said wave guide, the transverse external dimensions of said coupler being sufficiently small compared to the internal transverse dimensions of said waveguide that no substantial interference with transmission of the TE fundamental mode from said horn to said waveguide occurs at any communication frequency, said coaxial coupler including means in the horn thereof for converting TM horn signal components of said beacon signals into TEM signal components, coaxial line means connected to said coaxial coupler and extending through a wall of said antenna, said coaxial line means providing a TEM output separate from the TE output of said antenna, and means in the horn portion thereof for extracting TE horn signal components and coupling them to a second coaxial line means providing a TE output separate from the TE output of said an the throat of said antenna at the juncture of said horn and said waveguide, said horn and waveguide being adapted to operate on TE, signal energy and to produce in response to non-axial components TE and TM, signal energy, said coupler having elements responsive to said non-axial horn signal components and comprising an elongated substantially cylindrical conductive member having a diameter small compared to the cross-section of said waveguide and wherein both ends of said coupler are tapered.

7. The antenna of claim 6 wherein a portion of said coupler extends into the throat of said horn and contains longitudinal irises responsive to said TE signal energy.

8. The antenna of claim 6 wherein a portion of said coupler extends along said waveguide and terminates in 

1. An antenna system comprising: a beam steering system, a feed horn, a waveguide transmission line connected to said feed horn, a coaxial coupler mounted at the juncture of said waveguide and said horn, said coupler containing elements responsive to nonaxial horn signal components, said horn and waveguide being adapted to operate on TE11 signal energy, said non-axial components comprising a TE01 component when the departure from the axis is perpendicular to the polarization of said signal and a TM01 component when the departure from the axis is parallel to the polarization of said signal, said coupler including a plurality of longitudinal irises responsive to said TE01 signal, said irises being located at the end of the coupler extending into said horn, said coupler also including an axial coupling probe responsive to said TM01 signal components and extending along the axis of and in direction of said waveguide, and means for utilizing said non-axial components to operate said steering system to reduce said non-axial components.
 2. In an antenna of the type designed to receive both communications and beacon signals from a satellite, wherein said antenna includes means responsive to said beacon signals which, through the agency of antenna control servomechanisms, orient said antenna to track said satellite, said antenna including a horn section and a waveguide feed section, the improvement comprising: a coaxial coupler mounted at the juncture of said horn and said waveguide, said coaxial coupler including means extending into the throat of said horn section for converting TM01 horn signal components to TEM signal components, and means connected to said coupler for extracting said TEM signal components, said components being used to establish signals for operating said servomechanisms.
 3. The improvement of claim 2 wherein said coupler is tapered at both ends to minimize signal transmission discontinuities.
 4. The improvement of claim 2 wherein said means for extracting said TEM signal components includes an elongated probe oriented axially with respect to said waveguide feed section, said probe being connected to the center conductor of a coaxial cable passing through the interior of said coupler and through the wall of said waveguide feed section.
 5. An antenna for use in a satellite communications and tracking ground station where the satellite to be tracked transmits communications signals at one frequency and beacon signals at a second frequency, said antenna comprising a horn section to receive both said signals and a waveguide section coupled to said horn section for transmitting received signals to signal utilization means, a coaxial coupler mounted within said horn and waveguide sections at the juncture thereof, one portion of said coaxial coupler extending coaxially into the throat of said horn and another portion of said coaxial coupler extending coaxially into said waveguide, the transverse external dimensions of said coupler being sufficiently small compared to the internal transverse dimensions of said waveguide that no substantial interference with transmission of the TE11 fundamental mode from said horn to said waveguide occurs at any communication frequency, said coaxial coupler including means in the horn thereof for converting TM01 horn signal components of said beacon signals into TEM signal components, coaxial line means connected to said coaxial coupler and extending through a wall of said antenna, said coaxial line means providing a TEM output separate from the TE11 output of said antenna, and means in the horn portion thereof for extracting TE01 horn signal components and coupling them to a second coaxial line means providing a TE01 output separate from the TE11 output of said antenna.
 6. A receiving antenna comprising a horn section, a waveguide section, and a coupler mounted coaxially in the throat of said antenna at the juncture of said horn and said waveguide, said horn and waveguide being adapted to operate on TE11 signal energy and to produce in response to non-axial components TE01 and TM01 signal energy, said coupler having elements responsive to said non-axial horn signal components and comprising an elongated substantially cylindrical conductive member having a diameter small compared to the cross-section of said waveguide and wherein both ends of said coupler are tapered.
 7. The antenna of claim 6 wherein a portion of said coupler extends into the throat of said horn and contains longitudinal irises responsive to said TE01 signal energy.
 8. The antenna of claim 6 wherein a portion of said coupler extends along said waveguide and terminates in an axial probe responsive to said TM01 signal energy. 